Internal pressure in a molecular crystal under influence of light: the interplay with the structural phase transition

Abstract

The molecular crystal [Fe(ptz)₆](BF₄)₂, where ptz is 1-n-propyltetrazole, is a member of family of spin transition complexes. It shows a thermal spin transition from the highest spin multiplicity state (high-spin, S = 2) to the lowest multiplicity state (low-spin, S = 0). The spin transition is a matter of ligand field strength for this iron(II) complex, where iron is surrounded by nitrogen atoms. Owing to the important difference in radii between metal and ligand in the two aforementioned states of complex, one observes a difference of ∼5% in crystal volume. It follows that the spin transition may be accompanied by the crystallographic phase transition as well. At low temperatures, one can induce the spin transition of the complex by irradiating the crystal with the blue laser light. On the other hand, upon cooling, the high-spin state can be stabilised by red light. Recently, we obtained a spin density map of the photoexcited crystal, where the major part of spin distribution is situated around iron atoms. This information gives us an important structural parameter in the photoexcited metastable high-spin state of an iron-propyltetrazole single crystal.

Keywords:

• Internal pressure
• Photoexcitation
• Spin transition